1. Field of the Invention
The present invention relates to magnetic memory elements, and more specifically, to magnetic tunnel junction structures having reduced demagnetization coupling between pinned and free ferromagnetic layers for use in a magnetic random access memory (MRAM) device.
2. Brief Description of the Related Art
Various types of digital memory are used extensively in digital systems such as microprocessor-based systems, digital processing systems, and the like. Recently, magnetic random access memory (MRAM) devices have been investigated for possible use in non-volatile random access memory. The resistance of such a device changes based on the relative magnetized state of a sense (free) ferromagnetic layer to a pinned ferromagnetic layer. The magnetic moment of the pinned layer remains fixed while the magnetic moment of the free layer can change depending on an applied magnetic field. The relative magnetic direction of the free layer to the pinned layer are referred to as xe2x80x9cparallelxe2x80x9d and xe2x80x9cantiparallelxe2x80x9d.
Typically, a magnetic memory element, such as a magnetic tunnel junction (MTJ) memory element, has a structure that includes the free and pinned ferromagnetic layers separated by a non-magnetic tunnel junction barrier layer. These magnetic memory elements are formed using thin-film materials.
In response to parallel and antiparallel magnetic states, the magnetic memory element represents two different resistances to a current provided across the memory element in a direction perpendicular to the plane of the ferromagnetic layers. The resistance has minimum and maximum values corresponding to when the magnetization vectors of the free and pinned layers are parallel and antiparallel, respectively. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs across the barrier junction between the two separated sets of ferromagnetic layers.
Magnetic memory elements structurally include very thin layers, some of which are tens of angstroms thick. Due to the very thin layers and small size of the element, the magnetic field response of the free layer is affected by magnetic coupling between the free and pinned layers. Consequently, the magnetic vector of the free layer, for example, may preferentially orient in the antiparallel direction. This may destabilize the memory element and also make it more difficult to switch the magnetic vector of the free layer to the parallel direction.
A representative prior art MTJ structure is shown in FIG. 1. The structure includes a free ferromagnetic layer 10 separated from a pinned ferromagnetic layer 12 by a tunnel barrier layer 6. The free and pinned ferromagnetic layers may each be formed as a plurality of stacked individual layers. A seed layer 8 is typically provided below the free ferromagnetic layer 10. Pinned ferromagnetic layer 12 is pinned by an antiferromagnetic pinning layer 14. A cap layer 16 is also typically provided.
A disadvantage of the prior art MTJ structure shown in FIG. 1 is that demagnetizing coupling occurs between pinned layer 12 and free layer 10, as indicated by the curved arrows. As a result, in the absence of an applied external field, the magnetism of free layer 10 will tend to want to orient under the coupling influence of pinned layer 12 in the anti parallel direction. Consequently, the free layer 10 has different magnetic field strength switching thresholds when going from an anti parallel state to a parallel state and vice versa. This produces a magnetic field offset required to switch the free layer 10 from one orientation to the other.
FIG. 2 illustrates another prior art MTJ structure in which a set of pinned ferromagnetic layers 32, 38 are produced by a xe2x80x9csyntheticxe2x80x9d antiferromagnet in which the two ferromagnetic layers 32 and 38 are separated by an anti-parallel coupling layer 36 made of ruthenium, for example. The coupling layer 36 enhances magnetic coupling between pinned layer 32 and ferromagnetic layer 38 which reduces the undesired magnetic coupling between pinned layer 32 and free layer 30. While helping to demagnetize the free layer 30 from the effects of pinned layer 32, additional layers are required in the memory device as a special anneal process is required.
An MTJ structure having reduced demagnetization coupling between pinned and free layers, without the need for synthetic antiferromagnets, is desirable.
The present invention utilizes an additional ferromagnetic film on top of an antiferromagnetic pinning film to reduce the demagnetization coupling between free and pinned ferromagnetic layers in MRAM devices. A magnetic tunnel junction memory element according to the present invention includes a pinned ferromagnetic layer, a free ferromagnetic layer, and a barrier layer separating the pinned stack from the free stack. The pinned ferromagnetic layer has a ping antiferromagnetic layer adjacent to it. An offset ferromagnetic layer is further provided on a side of the pinning antiferromagnetic layer opposite the pinned ferromagnetic layer. The offset ferromagnetic layer reduces demagnetization coupling between the free ferromagnetic layer and the pinned ferromagnetic layer.